The quiescent state of peripheral naive T lymphocytes was thought to be due to the lack of activation signals. Recent studies, however, have shown that T cell quiescence is not by default but actively maintained by both cell extrinsic and intrinsic mechanisms, about which little is known at the transcriptional level. Forkhead box (FOX) proteins comprise a large family of transcription factors with diverse functions in development, aging and cancer. Recently we discovered that transcription factor Foxp1 exerts essential cell-intrinsic regulation of the quiescence of naive T cells, providing direct evidence that mature T cell quiescence is actively maintained. Meanwhile, our new preliminary studies also start to reveal that Foxp1 transcriptional network plays an important role in agonist- induced T cell activation. In this proposal, we will combine various approaches to identify functional targets and molecular pathways regulated by Foxp1 in controlling T cell quiescence, and to define the critical roles of Foxp1 in antigen-induced T cell activation and responses in vivo. Knowledge obtained from these studies will provide information for the design of new therapeutic strategies that would manipulate T cell activation status for the treatment of autoimmune and infectious diseases, and aid in vaccine development. PUBLIC HEALTH RELEVANCE: Recent studies have shown that T cell quiescence is not by default but actively maintained by both cell extrinsic and intrinsic mechanisms, about which little is known at the molecular level. The research on understanding the transcriptional regulation of T cell quiescence by key factors will provide mechanistic insights to guide the design of new therapeutic strategies that would manipulate T cell activation status for the treatment of autoimmune and infectious diseases, and aid in vaccine development.